The coherent resonant coupling of two autoionizing states of magnesium by an intense electromagnetic field is investigated experimentally by means of ionization and energy-selected photoelectron spectra probed from the ground state of the atom. The resulting effect of a drastic reduction in the ionization of the system is interpreted as the splitting of the two states achieved through their mixing by the field. A complete theoretical treatment of the problem employing the density matrix formalism is presented, reproducing the effects observed and explaining the phenomenon in terms of the relative populations of the states in the system.
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